1. Field of the Invention
The present invention relates to a surface acoustic wave resonator, a surface acoustic wave filter, a duplexer, and a communications apparatus including the same. More particularly, the present invention relates to a surface acoustic wave resonator using a surface acoustic wave including an SH wave as the main component.
2. Description of the Related Art
Conventionally, surface acoustic wave resonators have been used widely in band pass filters and other electronic components included in mobile communications equipment. As an example of such a surface acoustic wave resonator, a surface acoustic wave resonator or a surface acoustic wave filter having a configuration wherein an interdigital transducer (hereinafter referred to as IDT) including a comb-shaped electrode is formed on a piezoelectric substrate is well known. Since a surface acoustic wave including an SH wave as the main component, such as a Love wave, a leaky wave, a BGS wave, or the like can utilize edge reflection in such a surface acoustic wave resonator or surface acoustic wave filter, it is used practically in a small size resonator or surface acoustic wave filter, which does not require a reflector. Further, a piezoelectric single crystal, such as lithium niobate, lithium tantalate, or the like, is used as the material for the piezoelectric substrate of a surface acoustic wave resonator or a surface acoustic wave filter. In order to generate a surface acoustic wave including an SH wave as the main component, a 41.degree. Y cut X propagation substrate or a 64.degree. Y cut X propagation substrate needs to be used in the case of a lithium niobate, and a 36.degree. Y cut X propagation substrate needs to be used in the case of a lithium tantalate. The 41.degree. Y cut X propagation corresponds with the Euler angle indication of (0.degree., 131.degree., 0.degree.), the 64.degree. Y cut X propagation corresponds with the Euler angle indication of (0.degree., 154.degree., 0.degree.), and the 36.degree. Y cut X propagation corresponds with the Euler angle indication of (0.degree., 126.degree., 0.degree.).
However, the temperature coefficient of group delay time temperature characteristic (hereinafter referred to as TCD) of these piezoelectric single crystals is not good. More specifically, the TCD of a 41.degree. Y cut X propagation lithium niobate substrate is 80 ppm/.degree. C., the TCD of a 64.degree. Y cut X propagation lithium niobate substrate is 81 ppm/.degree. C., and the TCD of a 36.degree. Y cut X propagation lithium tantalate substrate is 32 ppm/.degree. C.
In general, in order to reliably obtain good characteristics in a surface acoustic wave resonator, a material with a good TCD is needed. That is, a material with a small frequency characteristic change according to the temperature change is needed. Therefore, if a surface acoustic wave resonator for generating a surface acoustic wave including an SH wave as the main component is provided with the lithium niobate or the lithium tantalate used as the substrate material as mentioned above, a problem arises in that the frequency characteristic shifts drastically. Moreover, even though lithium tantalate has a TCD which is better than that of lithium niobate, it also experiences the same problem with the frequency characteristic shift.
For example, in a surface acoustic wave resonator having a 100 MHz center frequency, a 50.degree. C. temperature change causes a 400 KHz frequency characteristic shift in the case of a 41.degree. Y cut X propagation lithium niobate substrate, a 405 KHz frequency characteristic shift is generated in the case of a 64.degree. Y cut X propagation lithium niobate substrate, and a 160 KHz frequency characteristic shift is generated in the case of a 36.degree. Y cut X propagation lithium tantalate substrate.
In order to prevent such a frequency characteristic shift, a temperature compensation circuit has been connected to a surface acoustic wave resonator. However, a problem arises in that the device as a whole including the surface acoustic wave resonator becomes bulky because of the added temperature compensation circuit so that it is difficult to achieve a small size component.